1. Field of the Invention
The present invention relates to a lead frame having an improved plating layer and a method for manufacturing the same.
2. Description of the Related Art
As shown in FIG. 1, Silver(Ag) plating is selectively performed on a coined part 11 and a pad part 12, where a chip is mounted on a conventional lead frame 10. To enhance soldering properties after resin molding in a semiconductor package process, solder plating is performed on an outer lead 13 of the lead frame 10 using tin-lead (Sn--Pb). The wet process required for solder plating the resin-molded structure causes deterioration in the reliability of the lead frame.
To solve the above problem, a method for pre-coating a lead frame 20 has been developed with a plating layer having good solder wettability before the semiconductor packaging process. As shown in FIG. 2, the lead frame material, a copper (Cu) sheet 21, is coated with a nickel (Ni) layer 22. A palladium (Pd)--Ni alloy layer 23 is coated on the Ni layer 22. A Pd layer 24 is coated on the Pd--Ni alloy layer 23.
The Ni layer 22 is formed using a plating solution based on nickel(II) sulfamate tetrahydrate (Ni(H.sub.2 NSO.sub.3).sub.2.4H.sub.2 O). The nickel (II) sulfamate tetrahydrate is a Ni salt of amid sulfonic acid available in the form of a crystal or oversaturated nickel(II) sulfamate tetra hydrate solution, which has a high solubility and increases the metal concentration of the plating bath. As the plating thickness increases, the internal stress and ductility of the metal sheet tend to decrease.
To solve the above problem, first and second additives of an organic compound are added to the Ni plating solution. The first additive is an auxiliary additive for controlling adsorption of a main polish, and enhancing crack-resistance of the plating layer. The first additive includes saccharine, naphthalene-1,3,6-trisulfonic acid trisodium salt hydrate (C.sub.10 H.sub.5 Na.sub.3 O.sub.9 S.sub.3) and naphthalene-1.5-disulfonic acid disodium salt (C.sub.10 H.sub.6 Na.sub.2 O.sub.6 S.sub.2), and each includes an .dbd.C--SO.sub.2 group.
A second additive is added because the first additive has no leveling effect for planarizing the plating surface. The second additive is a leveler, having strong adsorption, and is co-deposited with Ni to allow polishing and leveling of the plating. The second additive which is most widely used has C.dbd.O, C.dbd.C, C.tbd.C, C.dbd.N, C.tbd.N and N.dbd.N groups.
Also, an additional additive is a surface active agent for lowering the surface tension of the solution. This additional additive draws hydrogen (H.sub.2) bubbles away from the plating material to reduce the number of pits caused by the H.sub.2 bubbles.
However, the Ni layer 22 contains sulfur which is a main component of the first additive, and thus the chemical solubility is sharply increased. During annealing at 200.degree. C. or higher, the sulfur causes rearrangement of atoms, which causes brittleness of the plating layer. This causes brittleness of the Ni layer 22 as an indirect thermal effect of a process of assembling a semiconductor, and thereby causes cracking of the plating during a subsequent trimming and forming process.